Sleep advice tends to arrive as a single round number: eight hours. A new study out of Columbia University suggests the real target is both narrower and more consequential than that, and it applies to nearly every organ in the body, not just the brain.

Published in Nature, the research draws on roughly half a million participants in the UK Biobank and compares self-reported sleep duration against 23 separate “biological aging clocks” spanning 17 organ systems, from the liver and lungs to the heart, kidneys, and immune system. The result is a U-shaped curve: both too little and too much sleep track with faster biological aging across the body, with the healthiest aging pattern clustering around 6.4 to 7.8 hours a night.

Why 23 Clocks Instead of One

Most biological age estimates collapse the whole body into a single number. But organs don’t necessarily age in lockstep, ovarian function, for instance, can decline well ahead of other systems, which is part of why fertility has its own well-known biological clock. Study leader Junhao Wen and colleagues have been building organ-specific clocks instead, using machine learning applied to structural imaging data, blood proteins, and metabolic markers.

For an organ like the liver, that meant building separate clocks from protein data, metabolic data, and imaging data, letting the researchers check whether sleep’s association with aging shows up consistently across different biological layers, rather than being an artifact of any single measurement type.

A Body-Wide Pattern, Not Just a Brain One

Short sleep, under 6 hours, was linked not only to faster brain aging but also to depressive episodes, anxiety disorders, obesity, type 2 diabetes, hypertension, ischemic heart disease, and arrhythmias. Long sleep, over 8 hours, carried its own risks. Both extremes were associated with chronic obstructive pulmonary disease, asthma, and digestive conditions including gastritis and reflux disease.

Importantly, the authors are careful about causality here: the study can’t say that sleeping badly directly ages your organs, only that unusual sleep duration and faster biological aging travel together. Short or long sleep may simply be an early signal of poorer health elsewhere in the body, rather than a cause on its own.

Two Different Roads to the Same Depression

One of the more interesting threads in the paper concerns late-life depression. Using a statistical technique called mediation analysis, the team found that short sleep appears to connect to late-life depression fairly directly, while long sleep seems to act through a different route, specifically through biological aging in the brain and in fat tissue.

In other words, short sleepers and long sleepers who both end up depressed later in life may be arriving there by genuinely different biological paths, which, if confirmed, would argue against treating “abnormal sleep” as a single problem with a single fix.

The findings add to a growing picture in which sleep functions less like a simple lifestyle habit and more like a coordinating signal across the entire brain-body network — one that’s now measurable, organ by organ, rather than just inferred from how tired someone feels.

Original paper: O’Toole, C.K., Song, Z., Anagnostakis, F. et al. Sleep chart of biological ageing clocks in middle and late life. Nature (2026).